The present invention relates to circuitry and a method for converting a 50 Hz, 625 line (312.5 lines per field) PAL-type signal into and from 60 Hz, 525 line (262.5 lines per field) NTSC signals.
In the United States, television signals are formatted in accordance with an NTSC standard. In some foreign countries, television signals are formatted in according to a PAL standard, phase alternating line standard. Sometimes consumers want to play a PAL formatted tape through a video cassette recorder (VCR) and play that tape on an NTSC-type television set.
U.S. patent application Ser. No. 243,259, by C. Azar, et al., filed Sep. 12 1988, now U.S. Pat. No. 5,021,871, discloses a method and system for converting PAL television signals into a quasi-NTSC format. However, the system disclosed in the Azar patent does not handle frame conversion from the 50 hertz (Hz), 625 line PAL signal to the 60 Hz, 525 line NTSC format. Also, the Azar patent does not deal with the fact that the PAL signals have 625 lines per frame (312.5 lines per field) and the NTSC signal has 525 lines per frame (262.5 lines per field).
Prior art devices typically separate or demodulate the PAL-type video composite signal into chrominance and luminance signals. The chrominance signal is then applied to an analog to digital (A/D) converter which is independent of the A/D converter for the luminance signal channel. Additionally, the horizontal and vertical sync pulses for each field are separated from the color and the luminance information signals and discarded. At the output side, there is an independent sync generator to generate horizontal and vertical sync pulses, equalizing pulses, etc. The digital signals are read out of the memory using interpolation circuits, converted to analog and locked to the newly generated sync signals. These prior art devices store at least one frame (two fields) and, many times, several frames in order to permit the interpolation circuits to determine the differences between frame 1 and frame 2, and set the read out control commands properly. The prior art devices do not use FIFO RAMS. Thereafter, the separate signal channels are applied to multiple, independent D/A converters and then the analog signals are remixed together. These conversion systems are costly due to the number of components used.